This invention relates in general to thermal imaging compositions, and to lithographic imaging members (and particularly to lithographic printing plates) prepared therefrom. The invention also relates to a method of imaging such imaging members, and to a method of printing using them.
The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained by an imaged area and the water or fountain solution is preferentially retained by the non-imaged areas. When a suitably prepared surface is moistened with water and ink is then applied, the background or non-imaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The ink is then transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are becoming more common. Examples of such plates are described in U.S. Pat. No. 5,372,915 (Haley et al). They include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they require wet processing using alkaline developer solutions.
It has been recognized that a lithographic printing plate could be created by ablating an IR absorbing layer. For example, Canadian 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose). Such plates were exposed to focused near IR radiation with a Nd++ YAG laser. The absorbing layer converted the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying silicone rubber. Similar plates are described in Research Disclosure 19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates were developed by wetting with hexane and rubbing. Other publications describing ablatable printing plates include U.S. Pat. No. 5,385,092 (Lewis et al), U.S. Pat. No. 5,339,737 (Lewis et al), U.S. Pat. No. 5,353,705 (Lewis et al), U.S. Pat. No. Reissue 35,512 (Nowak et al), and U.S. Pat. No. 5,378,580 (Leenders).
While the noted printing plates used for digital, processless printing have a number of advantages over the more conventional photosensitive printing plates, there are a number of disadvantages with their use. The process of ablation creates debris and vaporized materials that must be collected. The laser power required for ablation can be considerably high, and the components of such printing plates may be expensive, difficult to coat, or unacceptable for resulting printing quality. Such plates generally require at least two coated layers on a support.
Thermally switchable polymers have been described for use as imaging materials in printing plates. By xe2x80x9cswitchablexe2x80x9d is meant that the polymer is rendered from hydrophobic to relatively more hydrophilic or, conversely from hydrophilic to relatively more hydrophobic, upon exposure to heat. U.S. Pat. No. 4,034,183 (Uhlig) describes the use of high powered lasers to convert hydrophilic surface layers to hydrophobic surfaces. A similar process is described for converting polyamic acids into polyimides in U.S. Pat. No. 4,081,572 (Pacansky). The use of high-powered lasers is undesirable in the industry because of their high electrical power requirements and because of their need for cooling and frequent maintenance.
U.S. Pat. No. 4,634,659 (Esumi et al) describes imagewise irradiating hydrophobic polymer coatings to render exposed regions more hydrophilic in nature. While this concept was one of the early applications of converting surface characteristics in printing plates, it has the disadvantages of requiring long UV light exposure times (up to 60 minutes), and the plate""s use is in a positive-working mode only.
U.S. Pat. No. 4,405,705 (Etoh et al) and U.S. Pat. No. 4,548,893 (Lee et al) describe amine-containing polymers for photosensitive materials used in non-thermal processes. Thermal processes using polyamic acids and vinyl polymers with pendant quaternary ammonium groups are described in U.S. Pat. No. 4,693,958 (Schwartz et al). U.S. Pat. No. 5,512,418 (Ma) describes the use of polymers having cationic quaternary ammonium groups that are heat-sensitive. However, the materials described in this art require wet processing after imaging.
WO 92/09934 (Vogel et al) describes photosensitive compositions containing a photoacid generator and a polymer with acid labile tetrahydropyranyl or activated ester groups. However, imaging of these compositions converts the imaged areas from hydrophobic to hydrophilic in nature.
In addition, EP-A 0 652 483 (Ellis et al) describes lithographic printing plates imageable using IR lasers, and which do not require wet processing. These plates comprise an imaging layer that becomes more hydrophilic upon imagewise exposure to heat. This coating contains a polymer having pendant groups (such as t-alkyl carboxylates) that are capable of reacting under heat or acid to form more polar, hydrophilic groups. Imaging such compositions converts the imaged areas from hydrophobic to relatively more hydrophilic in nature, and thus requires imaging the background of the plate, which is generally a larger area. This can be a problem when imaging to the edge of the printing plate is desired.
Copending and commonly assigned U.S. Ser. No. 09/162,905, filed on Sep. 29, 1998 by Leon, Underwood and Fleming, U.S. Ser. No. 09/163,020 filed on Sep. 29, 1998 by Leon, Underwood, Fleming and DeBoer, and U.S. Ser. No. 09/156,833 filed on Sep. 18, 1998 are directed to processless direct write printing plates that include an imaging layer containing heat sensitive polymers. The polymer coatings are sensitized to infrared radiation by the incorporation of an infrared absorbing material such as an organic dye or a fine dispersion of carbon black. Upon exposure to a high intensity infrared laser, light absorbed by the organic dye or carbon black is converted to heat, thereby promoting a physical change in the polymer (usually a change in hydrophilicity or hydrophobicity). The resulting printing plates can be used on conventional printing presses to provide, for example, negative images. Such printing plates have utility in the evolving xe2x80x9ccomputer-to-platexe2x80x9d printing market.
Some of the heat-sensitive polymers described in the copending applications, particularly the polymers containing organoonium or other charged groups, have a tendency to undergo physical interactions or chemical reactions with the organic dye or carbon black, thus compromising the effectiveness of both polymers and heat-absorbing materials. In particular, while carbon black is an infrared radiation absorbing material of preference because of its low cost and absorption of light throughout the infrared region of the electromagnetic spectrum, its use also creates problems. For example, it cannot be readily dispersed out of water or the alcoholic solvents of choice. Special carbon black products that are designed to be water-dispersible (that is, have special surface functionalities), however, often agglomerate in the presence of polymers (including organoonium polymers) containing ionic groups due to chemical interactions.
Thus, the graphic arts industry is seeking an alternative means for providing a processless, direct-write lithographic imaging members that can be imaged without ablation, or the other problems noted above in relation to known processless direct write printing plates. It would also be desirable to have heat-sensitive imaging members that include components that are highly effective to convert light exposure into heat and that can be coated out of water or other environmentally suitable solvents without agglomeration.
The problems noted above are overcome with a composition useful for thermal imaging comprising:
a) a hydrophilic heat-sensitive ionomer,
b) polymer grafted carbon, and
c) water or a water-miscible organic solvent.
This invention also provides an imaging member comprising a support and having disposed thereon a hydrophilic heat-sensitive layer that is prepared from the composition described above.
Still further, this invention includes a method of imaging comprising the steps of:
A) providing the imaging member described above, and
B) imagewise exposing the imaging member to provide exposed and unexposed areas in the imaging layer of the imaging member, whereby the exposed areas are rendered more hydrophobic than the unexposed areas by heat provided by the imagewise exposure.
Still again, a method of printing comprises the steps of carrying out steps A and B noted above, and additionally:
C) contacting the imaging member with a fountain solution and a lithographic printing ink, and imagewise transferring that printing ink from the imaging member to a receiving material.
As used herein, the term xe2x80x9cionomerxe2x80x9d refers to a charged polymer having at least 20 mol % of the recurring units negatively or positively charged. These ionomers are generally referred to as xe2x80x9ccharged polymersxe2x80x9d in the following disclosure.
The imaging members of this invention have a number of advantages, and avoid the problems of previous printing plates. Specifically, the problems and concerns associated with ablation imaging (that is, imagewise removal of a surface layer) are avoided because the hydrophilicity of the imaging layer is changed imagewise by xe2x80x9cswitchingxe2x80x9d (preferably, irreversibly) exposed areas of its printing surface to be less hydrophilic (that is, become more hydrophobic when heated). Thus, the imaging layer stays intact during and after imaging (that is, no ablation is required). These advantages are achieved by using a hydrophilic heat-sensitive polymer having recurring ionic groups within the polymer backbone or chemically attached thereto. Such polymers and groups are described in more detail below. The polymers used in the imaging layer are readily prepared using procedures described herein, and the imaging members of this invention are simple to make and use without the need for post-imaging wet processing. The resulting printing members formed from the imaging members of this invention are generally negative-working. In some cases, the polymers are crosslinked upon exposure and provide increased durability to the imaging members. In other and preferred cases, the polymers are crosslinked upon application to a support and curing.
Positively charged polymers, such as organoonium polymers that are preferred in the practice of this invention are typically coated out of water and methanol, solvents that readily dissolve these water-soluble polymeric salts. Carbon black does not readily disperse in such solvents and in order to achieve aqueous dispersions, special concentrated carbon black products are required (for example, materials sold by Cabot Laboratories under the trade name CAB-O-JET).
We have found, however, that charged polymers, such as organoonium salt polymers and such ionic functionalized carbon particles are not compatible with one another. The present invention solves this problem with the use of polymer grafted carbons as the infrared absorbing material instead of conventional dispersed carbon particles. These materials are also known in the art as carbon black-grafted polymers. We have found that they are readily dispersed in water or water-miscible solvents such as alcohols, and that they do not agglomerate in the presence of charged polymers. The compositions of this invention can then be readily applied to suitable metal or polymer supports, and the resulting imaging members function as improved thermal processless printing plates.
Polymer grafted carbon also provides an advantage of sensitization in that it provides greater wavelength independence. That is, it provides for light absorption throughout the infrared region of the electromagnetic spectrum, and specifically over the wavelength range at which commercial thermal platesetters operate.
The imaging members of this invention comprise a support and one or more layers thereon that include a dried heat-sensitive composition. The support can be any self-supporting material including polymeric films, glass, ceramics, cellulosic materials (including papers), metals or stiff papers, or a lamination of any of these materials. The thickness of the support can be varied. In most applications, the thickness should be sufficient to sustain the wear from printing and thin enough to wrap around a printing form. A preferred embodiment uses a polyester support prepared from, for example, polyethylene terephthalate or polyethylene naphthalate, and having a thickness of from about 100 to about 310 xcexcm. Another preferred embodiment uses aluminum sheets having a thickness of from about 100 to about 600 xcexcm. The support should resist dimensional change under conditions of use.
The support may also be a cylindrical support that includes printing cylinders on press as well as printing sleeves that are fitted over printing cylinders. The use of such supports to provide cylindrical imaging members is described in U.S. Pat. No. 5,713,287 (Gelbart). The heat-sensitive polymer composition can be coated or sprayed directly onto the cylindrical surface that is an integral part of the printing press.
The support may be coated with one or more xe2x80x9csubbingxe2x80x9d layers to improve adhesion of the final assemblage. Examples of subbing layer materials include, but are not limited to, gelatin and other naturally occurring and synthetic hydrophilic colloids and vinyl polymers (such as vinylidene chloride copolymers) that are known for such purposes in the photographic industry, vinylphosphonic acid polymers, sol gel materials such as those prepared from alkoxysilanes (including glycidoxypropyltriethoxysilane and aminopropyltriethoxysilane), epoxy functional polymers, and various ceramics.
The backside of the support may be coated with antistatic agents and/or slipping layers or matte layers to improve handling and xe2x80x9cfeelxe2x80x9d of the imaging member.
The imaging members, however, preferably have only one layer on the support, that is a heat-sensitive surface layer that is required for imaging. This hydrophilic layer is prepared from a composition of this invention, and dried to include one or more heat-sensitive charged polymers and a polymer grafted carbon as a photothermal conversion material (described below). Because of the particular polymer(s) used in the imaging layer, the exposed (imaged) areas of the layer are rendered more hydrophobic in nature. The unexposed areas remain hydrophilic in nature.
In the heat-sensitive imaging layer of the imaging member, only the one or more charged polymers and polymer grafted carbon are essential for imaging. The charged polymers generally are comprised of recurring units, of which at least 20 mol % include ionic groups. Preferably, at least 30 mol % of the recurring groups include ionic groups. Thus each of these polymers has a net charge provided by these ionic groups. Preferably, the ionic groups are cationic groups.
The charged polymers (ionomers) useful in the practice of this invention can be of three broad classes of materials:
I) crosslinked or uncrosslinked vinyl polymers comprising recurring units comprising positively-charged, pendant N-alkylated aromatic heterocyclic groups,
II) crosslinked or uncrosslinked polymers comprising recurring organoonium groups, and
III) polymers comprising a pendant thiosulfate (Bunte salt) group.
Each class of polymer is described in turn. The imaging layer can include mixtures of polymers from each class, or a mixture of one or more polymers of two or more classes. The Class II polymers are preferred.
Class I Polymers
The Class I polymers generally have a molecular weight of at least 1000 and can be any of a wide variety of hydrophilic vinyl homopolymers and copolymers having the requisite positively-charged groups. They are prepared from ethylenically unsaturated polymerizable monomers using any conventional polymerization technique. Preferably, the polymers are copolymers prepared from two or more ethylenically unsaturated polymerizable monomers, at least one of which contains the desired pendant positively-charged group, and another monomer that is capable of providing other properties, such as crosslinking sites and possibly adhesion to the support. Procedures and reactants needed to prepare these polymers are well known. With the additional teaching provided herein, the known polymer reactants and conditions can be modified by a skilled artisan to attach a suitable cationic group.
The presence of a cationic group apparently provides or facilitates the xe2x80x9cswitchingxe2x80x9d of the imaging layer from hydrophilic to hydrophobic in the areas that have been exposed to heat in some manner, when the cationic group reacts with its counterion. The net result is the loss of charge. Such reactions are more easily accomplished when the anion is more nucleophilic and/or more basic. For example, an acetate anion is typically more reactive than a chloride anion. By varying the chemical nature of the anion, the reactivity of the heat-sensitive polymer can be modified to provide optimal image resolution for a given set of conditions (for example, laser hardware and power, and printing press needs) balanced with sufficient ambient shelf life. Useful anions include the halides, carboxylates, sulfates, borates and sulfonates. Representative anions include, but are not limited to, chloride, bromide, fluoride, acetate, tetrafluoroborate, formate, sulfate, p-toluenesulfonate and others readily apparent to one skilled in the art. The halides and carboxylates are preferred.
The aromatic cationic group is present in sufficient recurring units of the polymer so that the heat-activated reaction described above can provide desired hydrophobicity of the imaged printing layer. The groups can be attached along a principal backbone of the polymer, or to one or more branches of a polymeric network, or both. The aromatic groups generally comprise 5 to 10 carbon, nitrogen, sulfur or oxygen atoms in the ring (at least one being a positively-charged nitrogen atom), to which is attached a branched or unbranched, substituted or unsubstituted alkyl group. Thus, the recurring units containing the aromatic heterocyclic group can be represented by the Structure I: 
In this structure, R1 is a branched or unbranched, substituted or unsubstituted alkyl group having from 1 to 12 carbon atoms (such as methyl, ethyl, n-propyl, isopropyl, t-butyl, hexyl, methoxymethyl, benzyl, neopentyl and dodecyl). Preferably, R1 is a substituted or unsubstituted, branched or unbranched alkyl group having from 1 to 6 carbon atoms, and most preferably, it is substituted or unsubstituted methyl group.
R2 can be a substituted or unsubstituted alkyl group (as defined above, and additionally a cyanoalkyl group, a hydroxyalkyl group or alkoxyalkyl group), substituted or unsubstituted alkoxy having 1 to 6 carbon atoms (such as methoxy, ethoxy, isopropoxy, oxymethylmethoxy, n-propoxy and butoxy), a substituted or unsubstituted aryl group having 6 to 14 carbon atoms in the ring (such as phenyl, naphthyl, anthryl, p-methoxyphenyl, xylyl, and alkoxycarbonylphenyl), halo (such as chloro and bromo), a substituted or unsubstituted cycloalkyl group having 5 to 8 carbon atoms in the ring (such as cyclopentyl, cyclohexyl and 4-methylcyclohexyl), or a substituted or unsubstituted heterocyclic group having 5 to 8 atoms in the ring including at least one nitrogen, sulfur or oxygen atom in the ring (such as pyridyl, pyridinyl, tetrahydrofuranyl and tetrahydropyranyl). Preferably, R2 is substituted or unsubstituted methyl or ethyl group.
Zxe2x80x3 represents the carbon and any additional nitrogen, oxygen, or sulfur atoms necessary to complete the 5- to 10-membered aromatic N-heterocyclic ring that is attached to the polymeric backbone. Thus, the ring can include two or more nitrogen atoms in the ring (for example, N-alkylated diazinium or imidazolium groups), or N-alkylated nitrogen-containing fused ring systems including, but not limited to, pyridinium, quinolinium, isoquinolinium acridinium, phenanthradinium and others readily apparent to one skilled in the art.
Wxe2x88x92 is a suitable anion as described above. Most preferably it is acetate or chloride.
Also in Structure I, n is 0 to 6, and is preferably 0 or 1. Most preferably, n is 0.
The aromatic heterocyclic ring can be attached to the polymeric backbone at any position on the ring. Preferably, there are 5 or 6 atoms in the ring, one or two of which are nitrogen. Thus, the N-alkylated nitrogen containing aromatic group is preferably imidazolium or pyridinium and most preferably it is imidazolium.
The recurring units containing the cationic aromatic heterocycle can be provided by reacting a precursor polymer containing unalkylated nitrogen containing heterocyclic units with an appropriate alkylating agent (such as alkyl sulfonate esters, alkyl halides and other materials readily apparent to one skilled in the art) using known procedures and conditions.
Preferred Class I polymers can be represented by the following Structure II: 
wherein X represents recurring units to which the N-alkylated nitrogen containing aromatic heterocyclic groups (represented by HETxe2x88x92) are attached, Y represents recurring units derived from ethylenically unsaturated polymerizable monomers that may provide active sites for crosslinking using any of various crosslinking mechanisms (described below), and Z represents recurring units derived from any additional ethylenically unsaturated polymerizable monomers. The various repeating units are present in suitable amounts, as represented by x being from about 20 to 100 mol %, y being from about 0 to about 20 mol %, and z being from 0 to 80 mol %. Preferably, x is from about 30 to about 98 mol %, y is from about 2 to about 10 mol % and z is from 0 to about 68 mol %.
Crosslinking of the polymers can be provided in a number of ways. There are numerous monomers and methods for crosslinking that are familiar to one skilled in the art. Some representative crosslinking strategies include, but are not necessarily limited to:
a) reacting an amine or carboxylic acid or other Lewis basic units with diepoxide crosslinkers,
b) reacting an epoxide units within the polymer with difunctional amines, carboxylic acids, or other difunctional Lewis basic unit,
c) irradiative or radical-initiated crosslinking of double bond-containing units such as acrylates, methacrylates, cinnamates, or vinyl groups,
d) reacting a multivalent metal salts with ligating groups within the polymer (the reaction of zinc salts with carboxylic acid-containing polymers is an example),
e) using crosslinkable monomers that react via the Knoevenagel condensation reaction, such as (2-acetoacetoxy)ethyl acrylate and methacrylate,
f) reacting an amine, thiol, or carboxylic acid groups with a divinyl compound (such as bis (vinylsulfonyl) methane) via a Michael addition reaction,
g) reacting a carboxylic acid units with crosslinkers having multiple aziridine units,
h) reacting a crosslinkers having multiple isocyanate units with amines, thiols, or alcohols within the polymer,
i) mechanisms involving the formation of interchain sol-gel linkages [such as the use of the 3-(trimethoxysilyl) propylmethacrylate monomer],
j) oxidative crosslinking using an added radical initiator (such as a peroxide or hydroperoxide),
k) autooxidative crosslinking, such as employed by alkyd resins,
l) sulfur vulcanization, and
m) processes involving ionizing radiation.
Monomers having crosslinkable groups or active crosslinkable sites (or groups that can serve as attachment points for crosslinking additives, such as epoxides) can be copolymerized with the other monomers noted above. Such monomers include, but are not limited to, 3-(trimethoxysilyl)propyl acrylate or methacrylate, cinnamoyl acrylate or methacrylate, N-methoxymethyl methacrylamide, N-aminopropylacrylamide hydrochloride, acrylic or methacrylic acid and hydroxyethyl methacrylate.
Additional monomers that provide the repeating units represented by xe2x80x9cZxe2x80x9d in the Structure II above include any useful hydrophilic or oleophilic ethylenically unsaturated polymerizable monomer that may provide desired physical or printing properties to the hydrophilic imaging layer. Such monomers include, but are not limited to, acrylates, methacrylates, isoprene, acrylonitrile, styrene and styrene derivatives, acrylamides, methacrylamides, acrylic or methacrylic acid and vinyl halides.
Representative Class I polymers are identified hereinbelow as Polymers 1 and 3-6. Mixtures of these polymers can also be used. Polymer 2 below is a precursor to a useful Class I polymer.
Class II Polymers
The Class II polymers also generally have a molecular weight of at least 1000. They can be any of a wide variety of vinyl or non-vinyl homopolymers and copolymers.
Non-vinyl polymers of Class II include, but are not limited to, polyesters, polyamides, polyamide-esters, polyarylene oxides and derivatives thereof, polyurethanes, polyxylylenes and derivatives thereof, silicon-based sol gels (solsesquioxanes), polyamidoamines, polyimides, polysulfones, polysiloxanes, polyethers, poly(ether ketones), poly(phenylene sulfide) ionomers, polysulfides and polybenzimidazoles. Preferably, such non-vinyl polymers are silicon based sol gels, polyarylene oxides, poly(phenylene sulfide) ionomers or polyxylylenes, and most preferably, they are poly(phenylene sulfide) ionomers. Procedures and reactants needed to prepare all of these types of polymers are well known. With the additional teaching provided herein, the known polymer reactants and conditions can be modified by a skilled artisan to incorporate or attach a suitable cationic organoonium moiety.
Silicon-based sol gels useful in this invention can be prepared as a crosslinked polymeric matrix containing a silicon colloid derived from di-, tri- or tetraalkoxy silanes. These colloids are formed by methods described in U.S. Pat. No. 2,244,325, U.S. Pat. No. 2,574,902 and U.S. Pat. No. 2,597,872. Stable dispersions of such colloids can be conveniently purchased from companies such as the DuPont Company. A preferred sol-gel uses N-trimethoxysilylpropyl-N, N,N-trimethylammonium acetate both as the crosslinking agent and as the polymer layer forming material.
The presence of an organoonium moiety that is chemically incorporated into the polymer in some fashion apparently provides or facilitates the xe2x80x9cswitchingxe2x80x9d of the imaging layer from hydrophilic to oleophilic in the exposed areas upon exposure to energy that provides or generates heat, when the cationic moiety reacts with its counterion. The net result is the loss of charge. Such reactions are more easily accomplished when the anion of the organoonium moiety is more nucleophilic and/or more basic, as described above for the Class I polymers.
The organoonium moiety within the polymer can be chosen from a trisubstituted sulfur moiety (organosulfonium), a tetrasubstituted nitrogen moiety (organoammonium), or a tetrasubstituted phosphorous moiety (organophosphonium). The tetrasubstituted nitrogen (organoammonium) moieties are preferred. This moiety can be chemically attached to (that is, pendant) the polymer backbone, or incorporated within the backbone in some fashion, along with the suitable counterion. In either embodiment, the organoonium moiety is present in sufficient repeating units of the polymer (at least 20 mol %) so that the heat-activated reaction described above can occur to provide desired hydrophobicity of the imaging layer. When chemically attached as a pendant group, the organoonium moiety can be attached along a principal backbone of the polymer, or to one or more branches of a polymeric network, or both. When chemically incorporated within the polymer backbone, the moiety can be present in either cyclic or acyclic form, and can also form a branching point in a polymer network. Preferably, the organoonium moiety is provided as a pendant group along the polymeric backbone. Pendant organoonium moieties can be chemically attached to the polymer backbone after polymer formation, or functional groups on the polymer can be converted to organoonium moieties using known chemistry. For example, pendant quaternary ammonium groups can be provided on a polymeric backbone by the displacement of a xe2x80x9cleaving groupxe2x80x9d functionality (such as a halogen) by a tertiary amine nucleophile. Alternatively, the organoonium group can be present on a monomer that is then polymerized or derived by the alkylation of a neutral heteroatom unit (trivalent nitrogen or phosphorous group or divalent sulfur group) already incorporated within the polymer.
The organoonium moiety is substituted to provide a positive charge. Each substituent must have at least one carbon atom that is directly attached to the sulfur, nitrogen or phosphorus atom of the organoonium moiety. Useful substituents include, but are not limited to, substituted or unsubstituted alkyl groups having 1 to 12 carbon atoms and preferably from 1 to 7 carbon atoms (such as methyl, ethyl, n-propyl, isopropyl, t-butyl, hexyl, methoxyethyl, isopropoxymethyl, substituted or unsubstituted aryl groups (phenyl, naphthyl, p-methylphenyl, m-methoxyphenyl, p-chlorophenyl, p-methylthiophenyl, p-N, N-dimethylaminophenyl, xylyl, methoxycarbonylphenyl and cyanophenyl), and substituted or unsubstituted cycloalkyl groups having 5 to 8 carbon atoms in the carbocyclic ring (such as cyclopentyl, cyclohexyl, 4-methylcyclohexyl and 3-methylcyclohexyl). Other useful substituents would be readily apparent to one skilled in the art, and any combination of the expressly described substituents is also contemplated.
The organoonium moieties include any suitable anion as described above for the Class I polymers. The halides and carboxylates are preferred.
Representative Class II non-vinyl polymers are identified herein below as Polymers 7-8 and 10. Mixtures of these polymers can also be used. Polymer 9 is a precursor to Polymer 10.
In addition, vinyl Class II polymers can be used in the practice of this invention. Like the non-vinyl polymers, such heat-sensitive polymers are composed of recurring units having one or more types of organoonium group. For example, such a polymer can have recurring units with both organoammonium groups and organosulfonium groups. It is also not necessary that all of the organoonium groups have the same alkyl substituents. For example, a polymer can have recurring units having more than one type of organoammonium group. Useful anions in these polymers are the same as those described above for the non-vinyl polymers. In addition, the halides and carboxylates are preferred.
The organoonium group is present in sufficient recurring units of the polymer so that the heat-activated reaction described above can occur to provide desired hydrophobicity of the imaged printing layer. The group can be attached along a principal backbone of the polymer, or to one or more branches of a polymeric network, or both. Pendant groups can be chemically attached to the polymer backbone after polymer formation using known chemistry. For example, pendant organoammonium, organophosphonium or organosulfonium groups can be provided on a polymeric backbone by the nucleophilic displacement of a pendant leaving group (such as a halide or sulfonate ester) on the polymeric chain by a trivalent amine, divalent sulfur or trivalent phosphorous nucleophile. Pendant onium groups can also be provided by alkylation of corresponding pendant neutral heteroatom groups (nitrogen, sulfur or phosphorous) using any commonly used alkylating agent such as alkyl sulfonate esters or alkyl halides. Alternatively a monomer precursor containing the desired organoammonium, organophosphonium or organosulfonium group may be polymerized to yield the desired polymer.
The organoammonium, organophosphonium or organosulfonium group in the vinyl polymer provides the desired positive charge. Generally, preferred pendant organoonium groups can be illustrated by the following Structures III, IV and V: 
wherein R is a substituted or unsubstituted alkylene group having 1 to 12 carbon atoms that can also include one or more oxy, thio, carbonyl, amido or alkoxycarbonyl groups with the chain (such as methylene, ethylene, isopropylene, methylenephenylene, methyleneoxymethylene, n-butylene and hexylene), a substituted or unsubstituted arylene group having 6 to 10 carbon atoms in the ring (such as phenylene, naphthylene, xylylene and 3-methoxyphenylene), or a substituted or unsubstituted cycloalkylene group having 5 to 10 carbon atoms in the ring (such as 1,4-cyclohexylene, and 3-methyl-1,4-cyclohexylene). In addition, R can be a combination of two or more of the defined substituted or unsubstituted alkylene, arylene and cycloalkylene groups. Preferably, R is a substituted or unsubstituted ethyleneoxycarbonyl or phenylenemethylene group. Other useful substituents not listed herein could include combinations of any of those groups listed above as would be readily apparent to one skilled in the art.
R3, R4 and R5 are independently substituted or unsubstituted alkyl group having 1 to 12 carbon atoms (such as methyl, ethyl, n-propyl, isopropyl, t-butyl, hexyl, hydroxymethyl, methoxymethyl, benzyl, methylenecarboalkoxy and a cyanoalkyl), a substituted or unsubstituted aryl group having 6 to 10 carbon atoms in the carbocyclic ring (such as phenyl, naphthyl, xylyl, p-methoxyphenyl, p-methylphenyl, m-methoxyphenyl, p-chlorophenyl, p-methylthiophenyl, p-N, N-dimethylaminophenyl, methoxycarbonylphenyl and cyanophenyl), or a substituted or unsubstituted cycloalkyl group having 5 to 10 carbon atoms in the carbocyclic ring (such as 1,3- or 1,4-cyclohexyl). Alternatively, any two of R3, R4 and R5 can be combined to form a substituted or unsubstituted heterocyclic ring with the charged phosphorus, sulfur or nitrogen atom, the ring having 4 to 8 carbon, nitrogen, phosphorus, sulfur or oxygen atoms in the ring. Such heterocyclic rings include, but are not limited to, substituted or unsubstituted morpholinium, piperidinium and pyrrolidinium groups for Structure V. Other useful substituents for these various groups would be readily apparent to one skilled in the art, and any combinations of the expressly described substituents are also contemplated.
Preferably, R3, R4 and R5 are independently substituted or unsubstituted methyl or ethyl groups.
Wxe2x88x92 is any suitable anion as described above for the Class I polymers. Acetate and chloride are preferred anions.
Polymers containing quaternary ammonium groups as described herein are most preferred vinyl Class II polymers.
In preferred embodiments, the vinyl Class II polymers useful in the practice of this invention can be represented by the following Structure VI: 
wherein Xxe2x80x2 represents recurring units to which the organoonium groups (xe2x80x9cORGxe2x80x9d) are attached, Yxe2x80x2 represents recurring units derived from ethylenically unsaturated polymerizable monomers that may provide active sites for crosslinking using any of various crosslinking mechanisms (described below), and Zxe2x80x2 represents recurring units derived from any additional ethylenically unsaturated polymerizable monomers. The various recurring units are present in suitable amounts, as represented by xxe2x80x2 being from about 20 to about 99 mol %, yxe2x80x2 being from about 1 to about 20 mol %, and zxe2x80x2 being from 0 to about 79 mol %. Preferably, xxe2x80x2 is from about 30 to about 98 mol %, yxe2x80x2 is from about 2 to about 10 mol % and zxe2x80x2 is from 0 to about 68 mol %.
Crosslinking of the vinyl polymer can be achieved in the same way as described above for the Class I polymers.
Additional monomers that provide the additional recurring units represented by Zxe2x80x2 in Structure VI include any useful hydrophilic or oleophilic ethylenically unsaturated polymerizable monomer that may provide desired physical or printing properties to the imaging layer. Such monomers include, but are not limited to, acrylates, methacrylates, acrylonitrile, isoprene, styrene and styrene derivatives, acrylamides, methacrylamides, acrylic or methacrylic acid and vinyl halides.
Representative vinyl polymers of Class II include Polymers 11-18 as identified herein below, and Polymer 14 is most preferred. A mixture of any two or more of these polymers can also by used.
Class III Polymers
Each of the Class III polymers has a molecular weight of at least 1000, and preferably of at least 5000. For example, the polymers can be vinyl homopolymers or copolymers prepared from one or more ethylenically unsaturated polymerizable monomers that are reacted together using known polymerization techniques and reactants. Alternatively, they can be addition homopolymers or copolymers (such as polyethers) prepared from one or more heterocyclic monomers that are reacted together using known polymerization techniques and reactants. Additionally, they can be condensation type polymers (such as polyesters, polyimides, polyamides or polyurethanes) prepared using known polymerization techniques and reactants. Whatever the type of polymers, at least 20 mol % (preferably 30 mol %) of the total recurring units in the polymer comprise the necessary heat-activatable thiosulfate groups.
The Class III polymers useful in the practice of this invention can be represented by the Structure VII wherein the thiosulfate group (or Bunte salt) is a pendant group: 
wherein A represents a polymeric backbone, R6 is a divalent linking group, and Y1 is hydrogen or a cation.
Useful polymeric backbones include, but are not limited to, vinyl polymers, polyethers, polyimides, polyamides, polyurethanes and polyesters. Preferably, the polymeric backbone is a vinyl polymer or polyether.
Useful R6 linking groups include xe2x80x94(COO)n(Z1)m,xe2x80x94 wherein n is 0 or 1, m is 0 or 1, and Z1 is a substituted or unsubstituted alkylene group having 1 to 6 carbon atoms (such as methylene, ethylene, n-propylene, isopropylene, butylenes, 2-hydroxypropylene and 2-hydroxy-4-azahexylene) that can have one or more oxygen, nitrogen or sulfur atoms in the chain, a substituted or unsubstituted arylene group having 6 to 14 carbon atoms in the aromatic ring (such as phenylene, naphthalene, anthracylene and xylylene), or a substituted or unsubstituted arylenealkylene (or alkylenearylene) group having 7 to 20 carbon atoms in the chain (such as p-methylenephenylene, phenylenemethylene-phenylene, biphenylene and phenyleneisopropylenephenylene). In addition, R6 can be an alkylene group, an arylene group, in an arylenealkylene group as defined above for Z1.
Preferably, R6 is an alkylene group of 1 to 3 carbon atoms, an arylene group of 6 carbon atoms in the aromatic ring, an arylenealkylene group of 7 or 8 carbon atoms in the chain, or xe2x80x94COO(Z1)mxe2x80x94 wherein Z1 is methylene, ethylene or phenylene. Most preferably, R6 is phenylene, methylene or xe2x80x94COOxe2x80x94.
Y1 is hydrogen, ammonium ion, or a metal ion (such as sodium, potassium, magnesium, calcium, cesium, barium, zinc or lithium ion). Preferably, Y1 is hydrogen, sodium ion or potassium ion.
As the thiosulfate group is generally pendant to the backbone, preferably it is part of an ethylenically unsaturated polymerizable monomer that can be polymerized using conventional techniques to form vinyl homopolymers of the thiosulfate-containing recurring units, or vinyl copolymers when copolymerized with one or more additional ethylenically unsaturated polymerizable monomers. The thiosulfate-containing recurring units generally comprise at least 20 mol % of all recurring units in the polymer, preferably they comprise from about 30 to 100 mol % of all recurring units. A polymer can include more than one type of repeating unit containing a thiosulfate group as described herein.
Polymers having the above-described thiosulfate group are believed to crosslink and to switch from hydrophilic thiosulfate to hydrophobic disulfide (upon loss of sulfate) with heating.
Thiosulfate-containing molecules (or Bunte salts) can be prepared from the reaction between an alkyl halide and thiosulfate salt as taught by Bunte, Chem. Ber. 7, 646, 1884. Polymers containing thiosulfate groups can either be prepared from functional monomers or from preformed polymers. Polymers can also be prepared from preformed polymers in a similar manner as described in U.S. Pat. No. 3,706,706 (Vandenberg). Thiosulfate-containing molecules can also be prepared by reaction of an alkyl epoxide with a thiosulfate salt, or between an alkyl epoxide and a molecule containing a thiosulfate moiety (such as 2-aminoethanethiosulfuric acid), and the reaction can be performed either on a monomer or polymer as illustrated by Thames, Surf. Coating, 3 (Waterborne Coat.), Chapter 3, pp. 125-153, Wilson et al (Eds.).
Representative synthetic methods for making ethylenically unsaturated polymerizable monomers and Class III polymers (Polymers 19-28) are illustrated below. Vinyl polymers can be prepared by copolymerizing monomers containing the thiosulfate functional groups with one or more other ethylenically unsaturated polymerizable monomers to modify polymer chemical or functional properties, to optimize imaging member performance, or to introduce additional crosslinking capability.
Useful additional ethylenically unsaturated polymerizable monomers include, but are not limited to, acrylates (including methacrylates) such as ethyl acrylate, n-butyl acrylate, methyl methacrylate and t-butyl methacrylate, acrylamides (including methacrylamides), an acrylonitrile (including methacrylonitrile), vinyl ethers, styrenes, vinyl acetate, dienes (such as ethylene, propylene, 1,3-butadiene and isobutylene), vinyl pyridine and vinylpyrrolidone. Acrylamides, acrylates and styrenes are preferred.
The imaging layer of the imaging member can include one or more Class I, II or III polymers with or without minor amounts (less than 20 weight %, based on total dry weight of the layer) of additional binder or polymeric materials that will not adversely affect its imaging properties.
In the composition used to provide the heat-sensitive layer, the amount of charged polymer is generally present in an amount of at least 1% solids, and preferably at least 2% solids. A practical upper limit of the amount of charged polymer in the composition is about 10% solids.
The amount of charged polymer(s) used in the imaging layer is generally at least 0.1 g/m2, and preferably from about 0.1 to about 10 g/m2 (dry weight). This generally provides an average dry thickness of from about 0.1 to about 10 xcexcm.
The imaging layer can also include one or more conventional surfactants for coatability or other properties, dyes or colorants to allow visualization of the written image, or any other addenda commonly used in the lithographic art, as long as the concentrations are low enough so they are inert with respect to imaging or printing properties.
It is essential that the heat-sensitive imaging layer includes one or more photothermal conversion materials to absorb appropriate radiation from an appropriate energy source (such as a laser), which radiation is converted into heat. Thus, such materials convert photons into heat phonons. Preferably, the radiation absorbed is in the infrared and near-infrared regions of the electromagnetic spectrum. The photothermal conversion materials useful in this invention are what are known in the art as polymer grafted carbon materials.
Thus, the term xe2x80x9cpolymer grafted carbonxe2x80x9d denotes a chemically modified carbon black in which a synthetic polymer or copolymer is covalently bonded to the surface of the carbon particles. Such modified carbons materials typically have improved dispersibility in specific solvents due to the steric stabilization imparted by the polymer grafts. It is well known in the art that specific surface functionalities can be introduced onto the surface of carbonaceous materials through a wide variety of known methods including electrical discharge treatment (JP 6-025572), treatment with ultraviolet radiation (JP 5-339516), chemical oxidation (FR-A-911,059, CA-A-537,787, FR-A-1,195,792, GB-A-895,990, U.S. Pat. No. 2,439,442 and U.S. Pat. No. 3,216,843), and through control of the pyrolytic conditions under which the carbon black is manufactured [xe2x80x9cCarbon Black,xe2x80x9d Marcel-Dekker, New York, 15 (1976)].
Numerous techniques have been reported for using these induced surface functionalities as points of attachment for the grafting of synthetic polymers. One common strategy includes the reaction of a carbon black containing a reactive surface group with a polymer or copolymer containing specific chemical moieties known to react with such a group (JP 9-059331 and JP 8-337624). One such example is the modification of a carboxylic acid functionalized carbon with a polymer or copolymer containing aziridine, oxazoline, thioepoxide, N-hydroxyalkylamide, or epoxide moieties (U.S. Pat. No. 4,880,857). Alternatively, a polymerization reaction can be carried out in the presence of a carbon black that is surface functionalized with chemical groups capable of initiating or terminating the polymerization. In JP 6-263830, such a procedure is reported in which a peroxide-functionalized carbon black is used to initiate the polymerization of vinylic monomers. Other such preparations are reported in JP 3-174422 and JP 56-129210.
Ionizing radiation (JP 59-152917 and JP 55-147561) and electrical treatment (JP 57-005711) have also been used to graft polymers onto a carbon surface.
The types of polymers that have been grafted to carbon include vinylic/acrylic polymers, condensation polymers (such as polyesters and polyurethanes), silicones, poly (alkylene oxides) and block, graft, and random copolymers and hybrids thereof (U.S. Pat. No. 4,880,857 and U.S. Pat. No. 4,940,749).
The polymer grafted carbon is generally present in the heat-sensitive imaging layer in an amount sufficient to provide an optical transmission density of at least 0.1, and preferably at least 0.4, at the operating wavelength of the imaging laser (for example, 830 nm). The particular amount needed for this purpose would be readily apparent to one skilled in the art, depending upon the specific material used. For example, the net amount of carbon present in the compositions of this invention in an amount of at least 0.1 %, preferably at least 0.2% solids, and generally up to 1% solids.
The amount of polymer grafted to the carbon may vary in the weight ratio of from about 0.2:1 to about 4:1, and preferably from about 0.2:1 to about 1:1.
Carbon particle size can also vary. Preferred carbon particles are greater than 0.1 xcexcm and less than 1 xcexcm. Particularly preferred particles are less than 0.5 xcexcm.
The heat-sensitive compositions and imaging layers can include additional photothermal conversion materials, although the presence of such materials is not preferred. Such optional materials can be dyes, pigments, evaporated pigments, semiconductor materials, alloys, metals, metal oxides, metal sulfides or combinations thereof, or a dichroic stack of materials that absorb radiation by virtue of their refractive index and thickness. Borides, carbides, nitrides, carbonitrides, bronze-structured oxides and oxides structurally related to the bronze family but lacking the WO2.9 component, are also useful. Useful absorbing dyes for near infrared diode laser beams are described, for example, in U.S. Pat. No. 4,973,572 (DeBoer). Particular dyes of interest are xe2x80x9cbroad bandxe2x80x9d dyes, that is those that absorb over a wide band of the spectrum.
Alternatively, the same or different photothermal conversion material (including a polymer grafted carbon) can be included in a separate layer that is in thermal contact with the heat-sensitive imaging layer. Thus, during imaging, the action of the additional photothermal conversion material can be transferred to the heat-sensitive imaging layer.
The heat-sensitive composition of this invention can be applied to a support using any suitable equipment and procedure, such as spin coating, knife coating, gravure coating, dip coating or extrusion hopper coating. In addition, the composition can be sprayed onto a support, including a cylindrical support, using any suitable spraying means for example as described in U.S. Pat. No. 5,713,287 (noted above).
The heat-sensitive compositions of this invention are generally formulated in and coated from water or water-miscible solvents including, but not limited to, water-miscible alcohols (for example, methanol, ethanol, isopropanol, 1-methoxy-2-propanol and n-propanol), methyl ethyl ketone, tetrahydrofuran, acetonitrile and acetone. Water, methanol, ethanol and 1-methoxy-2-propanol are preferred. Mixtures (such as a mixture of water and methanol) of these solvents can also be used if desired.
The imaging members of this invention can be of any useful form including, but not limited to, printing plates, printing cylinders, printing sleeves and printing tapes (including flexible printing webs), all of any suitable size or dimensions. Preferably, the imaging members are printing plates or on-press cylinders.
During use, the imaging member of this invention is exposed to a suitable source of energy that generates or provides heat, such as a focused laser beam or a thermoresistive head, in the foreground areas where ink is desired in the printed image, typically from digital information supplied to the imaging device. A laser used to expose the imaging member of this invention is preferably a diode laser, because of the reliability and low maintenance of diode laser systems, but other lasers such as gas or solid state lasers may also be used. The combination of power, intensity and exposure time for laser imaging would be readily apparent to one skilled in the art. Specifications for lasers that emit in the near-IR region, and suitable imaging configurations and devices are described in U.S. Pat. No. 5,339,737 (Lewis et al), incorporated herein by reference. The imaging member is typically sensitized so as to maximize responsiveness at the emitting wavelength of the laser. For carbon black sensitization, the carbon particle size may be selected to maximize absorption.
The imaging apparatus can operate on its own, functioning solely as a platemaker, or it can be incorporated directly into a lithographic printing press. In the latter case, printing may commence immediately after imaging, thereby reducing press set-up time considerably. The imaging apparatus can be configured as a flatbed recorder or as a drum recorder, with the imaging member mounted to the interior or exterior cylindrical surface of the drum.
In the drum configuration, the requisite relative motion between an imaging device (such as laser beam) and the imaging member can be achieved by rotating the drum (and the imaging member mounted thereon) about its axis, and moving the imaging device parallel to the rotation axis, thereby scanning the imaging member circumferentially so the image xe2x80x9cgrowsxe2x80x9d in the axial direction. Alternatively, the beam can be moved parallel to the drum axis and, after each pass across the imaging member, increment angularly so that the image xe2x80x9cgrowsxe2x80x9d circumferentially. In both cases, after a complete scan by the laser beam, an image corresponding to the original document or picture can be applied to the surface of the imaging member.
In the flatbed configuration, a laser beam is drawn across either axis of the imaging member, and is indexed along the other axis after each pass. Obviously, the requisite relative motion can be produced by moving the imaging member rather than the laser beam.
While laser imaging is preferred in the practice of this invention, imaging can be provided by any other means that provides or generates thermal energy in an imagewise fashion. For example, imaging can be accomplished using a thermoresistive head (thermal printing head) in what is known as xe2x80x9cthermal printingxe2x80x9d, described for example in U.S. Pat. No. 5,488,025 (Martin et al). Such thermal printing heads are commercially available (for example, as Fujisu Thermal Head FTP-040 MCS001 and TDK Thermal Head F415 HH7-1089).
Imaging of heat-sensitive compositions on printing press cylinders can be accomplished using any suitable means, for example, as taught in U.S. Pat. No. 5,713,287 (noted above), that is incorporated herein by reference.
After imaging, the imaging member can be used for printing without conventional wet processing. Applied ink can be imagewise transferred to a suitable receiving material (such as cloth, paper, metal, glass or plastic) to provide one or more desired impressions. If desired, an intermediate blanket roller can be used to transfer the ink from the imaging member to the receiving material. The imaging members can be cleaned between impressions, if desired, using conventional cleaning means.